Solid polymer fuel cell is a fuel cell using, as the electrolyte, a solid polymer such as ion exchange resin or the like. This fuel cell has a feature that its operating temperature is relatively low.
The solid polymer fuel cell, as shown in FIG. 1, has a basic structure in which the space formed inside cell partition walls 1a and 1b is divided by an assembly 10. The assembly 10 comprises a solid polymer electrolyte membrane 6 and a fuel diffusion electrode 4 and an oxidant diffusion electrode 5, each bonded to one side of the membrane 6. By dividing the space formed inside the cell partition walls 1a and 1b with the assembly 10, a fuel chamber 7 and an oxidant chamber 8 are formed inside the cell partition walls. The fuel chamber 7 communicates with the outside of cell via a fuel passage 2 formed in the cell partition wall 1a. The oxidant chamber 8 communicates with the outside of the cell via an oxidant passage 3 formed in the cell partition wall 1b. 
In the solid polymer fuel cell having the above basic structure, a fuel composed of hydrogen gas, methanol or the like is fed into the fuel chamber 7 via the fuel passage 2. Meanwhile, an oxygen-containing gas (to act as an oxidant) such as oxygen, air or the like is fed into the oxidant chamber 8 via the oxidant passage 3. When, in this state, an external load circuit (not shown) is connected to the diffusion electrodes 4 and 5, the fuel cell supplies an electric energy to the external circuit, according to the following reaction mechanism.
When a cation exchange electrolyte membrane is used as the solid polymer electrolyte membrane 6, the catalyst contained in the fuel diffusion electrode 4 contacts with a fuel, generating proton. The proton (hydrogen ion) passes through the solid polymer electrolyte membrane 6 and moves toward the oxidant chamber 8. The proton, which has reached the oxidant chamber 8, reacts with the oxygen of oxidant by the action of the catalyst contained in the oxidant diffusion electrode 5 to generate water. Meanwhile, the electron generated in the fuel diffusion electrode 4 together with the proton passes through the external load circuit and reaches the oxidant diffusion electrode 5. The external load circuit utilizes the energy generated in the above reaction mechanism, as an electric energy.
In the solid polymer fuel cell using, as the solid polymer electrolyte membrane, a cation exchange electrolyte membrane, a perfluorocarbonsulfonic acid resin membrane is used most commonly as the cation exchange electrolyte membrane.
In the cation exchange type fuel cell using such a perfluorocarbonsulfonic acid resin membrane, however, there is a problem that only a noble metal resistant in the acidic atmosphere is usable as the catalyst, since the reaction for power generation is conducted in a strongly acidic atmosphere. Further, a problem is pointed out that the perfluorocarbonsulfonic acid resin membrane is expensive, posing a limit in the reduction in the cost for fuel cell production.
In order to solve the above problems, it has been investigated to use a hydrocarbon-based anion exchange membrane in place of the perfluorocarbonsulfonic acid resin membrane, and several proposals have been made (Patent Literatures 1 to 4).
In the fuel cell using an anion exchange membrane, the field of reaction is basic. In general, metals are hardly dissolved in a basic atmosphere, unlike in an acidic atmosphere; therefore, metal catalysts other than noble metals are considered to be usable.
In the solid polymer fuel cell using an anion exchange membrane, the ion species moving through the solid polymer electrolyte membrane 6 differs from the ion species of a fuel cell using a cation exchange membrane. In the solid polymer fuel cell using an anion exchange membrane, the mechanism of generation of electric energy is as follows. That is, a fuel such as hydrogen, methanol or the like is fed into the fuel chamber and oxygen and water are fed into the oxidant chamber, whereby, in the oxidant diffusion electrode 5, the catalyst contained in the electrode contacts with the oxygen and the water, generating hydroxide ion (OH−). This hydroxide ion passes through the solid polymer electrolyte membrane 6 made of a hydrocarbon-based anion exchange membrane and reaches the fuel chamber 7, where the hydroxide ion reacts with the fuel fed to the fuel diffusion electrode 4, generating water and electron. The electron generated in the fuel diffusion electrode 4 passes through an external load circuit and reaches the oxidant diffusion electrode 5. The fuel cell utilizes the energy generated by the above reaction, as an electric energy.
Thus, the fuel cell using a solid polymer electrolyte membrane made of an anion exchange membrane differs in the ion species passing through the electrolyte membrane. Further, the fuel cell differs in the moving direction of the ion species. Furthermore, the fuel cell differs in the reaction in each electrode, from the case of using a cation exchange membrane (for example, water reacts at the oxidant electrode).
Hitherto, no sufficient study has been made on the difference in ion species and the movement of water, and no proposal has been made on any electrolyte membrane developed in consideration of a special reaction mechanism occurring in a fuel cell using an anion exchange membrane.
Patent Literature 1 discloses, as the anion exchange membrane for solid polymer electrolyte membrane, an anion exchange membrane comprising a porous membrane (e.g. woven fabric) and a hydrocarbon-based crosslinked polymer having an anion exchange group (e.g. quaternary ammonium salt group or quaternary pyridinium salt group), filled in the porous membrane.
Patent Literature 2 discloses an anion exchange membrane obtained by introducing a quaternary ammonium salt group into a hydrocarbon-based engineering plastic, followed by casting for membrane production.
Patent Literature 3 discloses an anion exchange membrane obtained by graft-polymerizing a hydrocarbon-based monomer having an anion exchange group, to a substrate composed of a fluorine-containing polymer.
In any of the Patent Literatures 1 to 3, the main aim is to solve the problem of reduction in methanol permeability, which problem is to be solved also in the direct methanol fuel cell using a cation exchange membrane.
In order to enhance the adhesion between a solid polymer electrolyte membrane composed of a cation or anion exchange membrane and a catalyst electrode layer, Patent Literature 4 proposes adhesion of a polymer having an ion exchange group of opposite polarity from the polarity of the ion exchange membrane onto the ion exchange membrane. As the ion exchange membrane, there is used an anion exchange membrane comprising a polyethylene porous film substrate and a hydrocarbon-based, crosslinked anion exchange resin filled in the substrate. In the Patent Literature 4, it is described that the ion exchange membrane obtained can be endowed with mechanical strength and flexibility and can have a low electric resistance by being made in a thin state. However, in the Patent Literature 4, there is no mention on a constitution based on the full consideration of the above-mentioned special reaction mechanism when an anion exchange membrane is used.
Patent Literature 1: JP 1999-135137 A
Patent Literature 2: JP 1999-273695 A
Patent Literature 3: JP 2000-331693 A
Patent Literature 4: JP 2007-42617 A